Well thats a pretty vague question....but I guess I can give a couple answers?
PS3 is the most powerful, followed by the 360, then the Wii.
PS3 will have the most exclusives and bigger game library
(including the most amount of RPGs, like always)
PS3 online play is free.
PS3 can play blu-rays.
Wii has the quarky new controller.(might make games more fun)
Wii will have the regular amount of party games.
Wii will have zelda, nuff said?
360 has Halo 3(some believe is reason enough to buy it alone(i do not))
360 is out now.
360 rates 2nd in performace....
So depending on what is important for you I would go with that.
Personally? I like RPGs alot, along with their exclusives(Devil May Cry, among others) and like the concept of free internet so I'm going with PS3. Theres a reason for grabbing every system, though. Choose whats best for you.
There is no real "best system", just the best system for YOU.
Edit: after reading one of the above comments, the PS3 price isnt really out of hand, because the 500$ system is VERY good, and will be the one I'm getting. the differences between the 500 and 600 versions are negligible unless you have an HDMI tv(which most of us dont). Also, the online play will be free, so after 2 years its the same price as Xbox 360 anyways, and will thus only go down from there. Wii's price is very very VERY reasonable, though.